1. Field of the Invention
This invention relates to circuits for providing a source of current and, more particularly, to a low-voltage precision current source for providing a current at the output thereof that is substantially independent to variations in the operating potential applied thereto.
2. Description of the Prior Art
The prior art is replete with different types of current source circuits for providing an output current that is a function of some parameter of the circuit. For instance, a basic current source that is well known in the art is the simple current mirror circuit comprising a diode-connected transistor having its emitter and base coupled in parallel with the emitter and base of a second transistor. The commonly connected emitters of each transistor are connected to a source of operating potential with the collector of the diode-connected transistor being coupled to a source of input current. The input current is mirrored through the second transistor wherein the current flowing in the collector thereof is equal in magnitude to the value of the input current. Moreover, as is understood, the value of the collector current flowing in the second transistor can be made any ratio of the input current by area ratioing the emitter areas of the two transistors. The disadvantage of this circuit is that errors are inherent therein which prevents the absolute matching of the output current to the input current. The most significant cause of this error is the base current error associated with the two transistors, especially if these devices are PNP transistors. Additionally, this circuit is subject to errors induced by power supply ripple variations due to finite output impedance.
Other prior art current sources reduce the errors associated with the PNP base current flow but suffer in that higher compliance voltages are required. Compliance voltage is defined as the voltage drop required to be developed across the current source in order to provide a current at the output thereof. Thus, for example, a prior art circuit which is a modification of the above described circuit, and which provides good current matching between the input and output currents, is generally known in the art as the "Wilson" type current source. Although reducing base current errors, the aforementiond circuit requires a minimum compliance voltage equal to a V.sub.be drop plus the voltage drop across the collector-to-emitter of a transistor operated in a saturated condition, where V.sub.be is the voltage drop across the base-to-emitter of the transistor. Thus, there are some applications, for example, where low operating potentials are required in conjunction with a requirement for maximum desired load voltages. This application would not allow such a current source to be utilized since the compliance voltage drop would prevent operation of the load circuit coupled thereto.
Thus, there is a need for a low-voltage precision current source suitable to be fabricated in monolithic integrated circuit form that provides an output current the magnitude of which is determined by a known and constant parameter of the current source and which is independent to ripple variations in the supplied power supply voltage.
Additionally, the need arises for such a current source which has reduced beta current error to therefore provide a good match between the input and output currents.
Additionally, the precision current source would require a minimum compliance voltage of only one V.sub.ce (sat).